1. Field of the Invention
The present invention relates to a chloroprene polymer composition having excellent extrusion processability, tensile strength, flexural strength and thermal stability.
2. Description of the Prior Arts
It has been proposed to blend a solvent soluble chloroprene (hereinafter referring to as sol chloroprene polymer) to a highly crosslinked gel chloroprene polymer in order to improve extrusion processability of chloroprene polymer. However, the physical properties of such compositions have not been satisfactory. That is, in the conventional process, when a content of the gel chloroprene polymer is high, the extrusion velocity can be increased and the an extrusion expansion (die swell) can be minimized in the extrusion processability. However, the precise shaping in its molding and the smoothness of the surface are inferior and the mechanical strength and the storage stability are remarkably inferior.
In order to overcome such disadvantages, it has been proposed to reduce the content of the gel chloroprene polymer or to incorporate a softening agent or a liquid chloroprene polymer prepared with large amounts of molecular weight modifier. However, in the former method, the extrusion processability as the basical characteristic is inferior whereas in the later method, the fluidity and the surface smoothness of the extruded product are improved, but the dimentional stability and the mechanical strength are inferior. It has been difficult to impart satisfactory extrusion processability, mechanical strength and thermal stability.
Various studies have been worked in order to improve the mechanical strength of chloroprene polymer composition for extrusion molding. For example, a method of incorporating a sol chloroprene polymer prepared by using a sulfur containing compound as a molecular weight regulator has been proposed (Japanese patent publication Nos. 16049/1973 and 53441/1975.) However, in such method, the content of the gel chloroprene polymer should be lowered from the required content in order to maintain the mechanical strength of the gel chloroprene polymer composition. The surface smoothness and the die swell among the extrusion processability have been unsatisfactory. Such chloroprene polymer compositions have the disadvantage of remarkably inferior thermal stability. It has been difficult to impart satisfactory extrusion processability, mechanical strength and thermal stability. It has not been found what kinds of a gel chloroprene polymer and a sol chloroprene polymer are suitable for imparting these desired characteristics.